Diagram of traditional Reptilia: it is not a clade. To be a clade it would need to include birds (Aves) and drop Amniota. This is why the term 'Sauropsida' is often preferred to the term 'Reptilia' (in modern taxonomy).

Reptile is the common name for one of the main groups of land vertebrates. It is not used so much by biologists, who use more accurate terms.

The name "reptile" comes from Latin and means "one who creeps". All living reptile species are cold blooded, have scaly skin, and lay cleidoic eggs. They excrete uric acid (instead of urea), and have a cloaca. A cloaca is a shared opening for the anus, urinary tract and reproductive ducts. Reptiles also share an arrangement of the heart and major blood vessels which is different from that of mammals.

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Birds in relation to reptiles

Some reptiles are more closely related to birds than they are to other reptiles. Crocodiles are more closely related to birds than they are to lizards. Theropoddinosaurs are even more closely related, because birds evolved from them.

Taxonomy

Reptilia is an evolutionary grade rather than a clade. The main reason is that the term 'reptile' does not include birds, the descendents of theropoddinosaurs. Another reason is that the word 'reptile' is misleading because many extinct types were very different from living reptiles.

So instead of Reptilia as a taxonomic class, today most experts use Class Sauropsida (which includes all reptiles and birds, living and extinct). Class Synapsida includes mammals and all their forebears. Reptile is still the usual informal term to describe living snakes and lizards. Mammals are a genuine clade, and so Mammalia is still the taxonomic term.

Since reptiles are not monophyletic, reclassifying them is one of the key aims of researchers. Some taxonomists, such as Benton, make Sauropsida and Synapsida as class-level taxa. The two groups split in the Carboniferous, from stem-group Amniotes (the early tetrapods, which laid cleidoic eggs).

Sustained energy output (joules) of a typical reptile versus a similar size mammal as a function of core body temperature. The mammal has a much higher peak output, but can only function over a very narrow range of body temperature.